1. Field of the Invention
The invention relates to the field of methods for zooming onto a part of an image which consists of pixels and which represents a terrain overflown by an aircraft. The terrain overflown by the aircraft is either the terrain actually overflown by the aircraft when the image is displayed, or the terrain which is intended to be overflown by the aircraft, the image then being displayed in anticipation. The zoom method is preferably carried out by certain functional blocks of a cartographic function of a cartographic accelerator card described in detail subsequently.
The image is in general a 2D5 cartographic image, that is to say one representing a terrain plan view which is modulated, pixel by pixel, by a shading cue representative of the terrain relief. The image may be an altimetric image or a planimetric image. This shading cue determines-the brightness of each pixel of the image considered. In the case of an unzoomed image, that is to say an image whose zoom factor equals one, this shading cue is determined by way of a computation which is carried out, in the reference frame of a display screen of the aircraft, pixel by pixel, on the basis of the altitudes of pixels neighboring the current pixel. These neighboring pixels are preferably pixels immediately neighboring the current pixel, that is to say the pixels closest to the current pixel from among the neighboring pixels. The shading cue is computed on the one hand on the basis of a model of illumination of the image using one or more light sources, point and/or diffuse, and on the other hand on the basis of the local slope at the level of the current pixel, that is to say each of the pixels of the image, this local slope being represented by the orientation of a facet at the level of the current pixel.
2. Description of Prior Art
According to the prior art, this computation can be undertaken according to several different techniques among which the most common are the technique of so-called “flat shading”, the technique of so-called “gouraud shading”, and the so-called “Phong” technique. All these techniques have in common the use of characteristics of the neighboring pixels, such as for example the orientation of the normals to the neighboring pixels for the so-called “gouraud shading” technique, to determine the orientation of the facet which represents the local slope at the level of the current pixel. These techniques give better or worse results depending on the technique, at the cost of greater or lesser complexibility in the implementation of said techniques. The results are generally correct for an unzoomed image. On the other hand, for a zoomed image, and especially for an image whose zoom factor becomes considerable, a facetization of the image referred to as “bathroom tiling” appears (except for the so-called “Phong” technique, but the latter is complex and expensive in terms of resources) which is an impediment to the pilot, especially in dynamic mode on account of the flashing which occurs at the boundary between the various “bathroom tiles” and on which the pilot's attention is focused.